9301309 Pianetta This proposal describes experiments designed to quantitatively determine the atomic structure of alkali metals on surfaces of silicon and germanium. Experimentally, emphasis will be placed on the soft x-ray standing wave (XSW) technique performed in conjunction with surface extended x-ray adsorption fine structure (SEXAFS). This will be followed with scanning tunneling microscopy (STM) to aid in the final structural determination. XSW provides the perpendicular distances between the substrate planes and the overlayer, while SEXAFS allows precise bond length determinations. Together these techniques can determine adsorption angles and hence interfacial geometries of complex metal-semiconductor systems. In combination with STM, these techniques form a very complimentary set of probes for the complete structural determination at an interface. The main goals of this proposal are to achieve an in- depth understanding of the interfacial atomic structure alkali metal overlayers and to correlate the information with their electronic properties. In addition, this structural and electronic information will test and further develop state-of-the-art theoretical calculations. %%% Novel electronic properties of interfaces have enormous commercial applications in electronics. The simplest such interface, used as a building block of most practical devices, is a junction between a metal and a semiconductor. In this proposal, the microscopic (atomic) structure of single layers of alkali metals (such as sodium, potassium) deposited on silicon and germanium will be studied. These layers display a unique ability to substantially lower the potential barrier for election escape from the semiconductor and, because of that, are used in infrared detectors and photocathodes. This investigation will use several techniques based on synchrotron radiation. In particular, a newly developed standing wave method which allows precise location of foreign atoms on surfa ces will be utilized. The results will be of fundamental interest and have potential to aid development of new materials for technological applications. ***
